ATXN1 protein family and CIC regulate extracellular matrix remodeling and lung alveolarization

Abstract

Although expansion of CAG repeats in ATAXIN1 (ATXN1) causes Spinocerebellar ataxia type 1, the functions of ATXN1 and ATAXIN1-Like (ATXN1L) remain poorly understood. To investigate the function of these proteins, we generated and characterized Atxn1L(-/-) and Atxn1(-/-); Atxn1L(-/-) mice. Atxn1L(-/-) mice have hydrocephalus, omphalocele, and lung alveolarization defects. These phenotypes are more penetrant and severe in Atxn1(-/-); Atxn1L(-/-) mice, suggesting that ATXN1 and ATXN1L are functionally redundant. Upon pursuing the molecular mechanism, we discovered that several Matrix metalloproteinase (Mmp) genes are overexpressed and that the transcriptional repressor Capicua (CIC) is destabilized in Atxn1L(-/-) lungs. Consistent with this, Cic deficiency causes lung alveolarization defect. Loss of either ATXN1L or CIC derepresses Etv4, an activator for Mmp genes, thereby mediating MMP9 overexpression. These findings demonstrate a critical role of ATXN1/ATXN1L-CIC complexes in extracellular matrix (ECM) remodeling during development and their potential roles in pathogenesis of disorders affecting ECM remodeling.

Figure 1

Developmental abnormalities in Atxn1 and Atxn1L double mutant mice. (A) Bar graph representation of the average body weight of Atxn1L^−/− and littermates at 3.5 weeks after birth. *P<0.05. Error bars show s.d. (B) Incidence of hydrocephalus in Atxn1L^−/− mutants after weaning age. Right panel shows hematoxylin and eosin (H&E) staining of formalin-fixed section of whole brains from hydrocephalic Atxn1L-null mouse and WT littermate at 9 weeks after birth. (C) Picture of Atxn1^−/−; Atxn1L^−/− and wild type neonates. (D) Hydrocephalus in Atxn1^−/−; Atxn1L^−/− mice. H&E staining of formalin-fixed section of whole brains from hydrocephalic Atxn1^−/−; Atxn1L^−/− and WT littermate at P0. (E) Abdominal wall closure defect in Atxn1^−/−; Atxn1L^−/− mice. Upper pannel: images showing the abdominal wall closure defect in the Atxn1^−/−; Atxn1L^−/− at E19. The arrow indicates protruding gut into the umbilical ring (omphalocoele). Middle panel: Abdominal anatomy on WT and Atxn1^−/−; Atxn1L^−/− neonates. Arrow indicates the midgut. Lower panel shows the remaining guts dissected from WT and the Atxn1^−/−; Atxn1L^−/− neonates.

Figure 2

Alveolarization defects in Atxn1L^−/− mice. (A) H&E staining of formalin-fixed section of lung tissues from WT, Atxn1^−/− and Atxn1L^−/− at either P6 or P17. The alveolarization defect causing enlargement of the air spaces is observed in Atxn1L^−/− mice at P17. (B) QRT-PCR analysis of levels of eight Mmp genes using total RNA from the lungs of either WT, Atxn1^−/− or Atxn1L^−/− at P6 (n=4 per each genotype). *P<0.05. #P=0.0542. All error bars show s.e.m. (C) Gelatin zymography and Western blot analysis showing increased Mmp9 levels in lung and meningial tissues from 6 day-old Atxn1L^−/− mice. Twenty μg of lung extract and 10 μg of meningial tissue extract were loaded in each lane. (D) QRT-PCR analysis for Timp1, Timp2 and Timp3 levels in the lungs from 6 day-old wild-type and Atxn1L^−/− mice (n=3 per each genotype). (E) Verhoeff staining for elastin of alveolar walls. Formalin-fixed section of lung tissues from 7-9 month-old WT and Atxn1L^−/− mice was used. Marked decrease in elastic fiber formation on alveolar walls is found in Atxn1L^−/− mice with the alveolarization defects, compared with WT. The upper pictures show 10X magnified images for Verhoeff staining. The lower pictures represent enlarged images (80X) for the boxed areas in the upper pictures. Arrows indicate elastic fibers. (F) Bar graph for quantification of percentage of elastic fibril positive area within alveolar walls (n=4 per each genotype). *P<0.05. All error bars show s.e.m.

Figure 3

Loss of Atxn1/Atxn1L-Cic complexes causes the alveolarization defects. (A) Western blot analysis for Atxn1, Atxn1L and Cic using several different tissues from E18.5 wild-type embryos. Twenty μg of protein was loaded in each lane. (B) Representative Western blot image showing the Cic levels in lung and brain tissues from each genotype at E18.5. (C) Quantitative analysis of the Cic levels in the lung from each genotype at E18.5 (n=3 per each genotype). *P<0.05. All error bars show s.e.m. (D) Representative Western blot image showing the Cic levels in lung tissues from each genotype at P6. (E) Quantitative analysis of the Cic levels in the lung tissues from each genotype at P6 (n=3 per each genotype). **P<0.01. All error bars show s.e.m. (F) Western blot image showing increased Mmp9 levels in lung tissues from Cic-L^−/− compared with Cic-L^+/− mice. (G) H&E staining of formalin-fixed lung tissues from the Cic-L^−/− and Cic-L^+/− mice at P20. Severe alveolarization defects are observed in Cic-L^−/− mice.

Figure 4

Derepression of Pea3 group genes by the loss of Atxn1 and Atxn1L. (A) Schematic diagram for the location of Cic binding sites (Blue arrow head: TGAATGAA, Red arrow head: TGAATGGA) in Pea3 group gene promoter regions. Black arrows indicate primers designed to amplify Pea3 group gene promoter regions containing Cic binding sites by PCR. Transcription start site (+1) of each gene was determined based on the following NCBI reference sequences (NM_007960.4 for Etv1, NM_008815.2 for Etv4 and NM_023794.2 for Etv5). (B) ChIP-PCR analysis showing Cic promoter occupancy of Pea3 group genes in P6 lung tissue. (C) QRT-PCR analysis of Etv1, Etv4 and Etv5 expression levels using total RNA from the lungs of WT, Atxn1^−/− and Atxn1L^−/− mice (P6, n=4 per each genotype). *P<0.05. All error bars show s.e.m. (D) Western blot image showing an increase in Etv4 protein levels in lung tissues from Atxn1L^−/− mice. In contrast, Etv4 protein levels are normal in Atxn1^−/− mice. (E) QRT-PCR analysis of levels of Etv1, Etv4 and Etv5 using total RNA from the lungs of embryos of each genotype (E18.5, n=4 per each genotype). *P<0.05 and **P<0.01. All error bars show s.e.m.

Figure 5

Derepression of Etv4 up-regulates Mmp9 levels in alveolar macrophage cells. (A) Western blot image showing a decrease in Mmp9 levels upon knock-down of Etv4 in MH-S cells. (B) QRT-PCR analysis for changes in Etv4 levels upon treatment of MH-S cells with four different combinations of siRNAs (negative control, Cic and Etv4). Three independent experiments were carried out. *P<0.05 and **P<0.01. All error bars show s.e.m. (C) Representative Western blot image for changes in Mmp9 and Cic levels upon treatment of MH-S cells with four different combinations of siRNAs (negative control, Cic and Etv4). (D) Quantitative analysis of Cic and Mmp9 levels based on Western blot images from (C). Three independent experiments were carried out. *P<0.05 and **P<0.01. All error bars show s.e.m.